How do I expand backslash-escape sequences in C-style quoted strings?

A key point when scanning quoted strings is that you cannot (easily) write
a single rule that will precisely match the string if you allow things
like embedded escape sequences and newlines. If you try to match strings
with a single rule then you’ll wind up having to rescan the string anyway
to find any escape sequences.

Instead you can use exclusive start conditions and a set of rules, one for
matching non-escaped text, one for matching a single escape, one for
matching an embedded newline, and one for recognizing the end of the
string. Each of these rules is then faced with the question of where to
put its intermediary results. The best solution is for the rules to
append their local value of yytext to the end of a “string literal”
buffer. A rule like the escape-matcher will append to the buffer the
meaning of the escape sequence rather than the literal text in yytext.
In this way, yytext does not need to be modified at all.